Conventionally, a television set has a screen with a particular aspect (width to height) ratio. This ratio is 4:3 for most televisions.
Some television signals have different aspect ratios, such as a D2-MAC broadcast via satellite, which has an aspect ratio of 16:9. By reducing the picture height by 25% (using only 75% of the entire screen height), a 16:9 aspect signal may be completely reproduced on a 4:3 aspect screen, with a 12.5% void area above and below the picture.
A television set which has the drive and/or cut off levels of its picture tube automatically adjusted, does this by inserting sampling pulses into some lines of the television signal where there is no useful picture information (sampling lines). A cathode current resulting from the sampling lines is measured, and the drive and cut off levels are adjusted such that the cathode current achieves a desired level. When displaying a 4:3 signal on a 4:3 screen, the sampling lines are above the topmost visible part of the screen, hence the lines are out of view.
A problem with this arrangement is that in the case of a 16:9 signal being viewed on a 4:3 screen, such sampling lines appear in the void area, causing disturbance to a person viewing the television screen.
This invention seeks to provide a vertical timebase circuit in which the above mentioned disadvantage has been mitigated.